The first related arts in which electrodes formed of conducting films are provided on the front face and the rear face of a dielectric substrate, is the generally known planar dielectric transmission line resonators (hereinafter referred to as the PDTL resonators) are formed in the electrodes on the both faces, and the PDTL resonators are each composed of rectangular apertures opposite to each other with the dielectric substrate being sandwiched therebetween (for example, Japanese Unexamined Patent Application Publication No. 11-4108). In such first related arts, adjoining two-stage resonators are formed on the same substrate and the resonators are coupled to each other to form a dielectric filter.
The second related known arts is which three or more stages of resonators (for example, PDTL resonators or TE010-mode resonators) are arranged in a line on the same substrate and the adjoining resonators are coupled to each other to form a dielectric filter (for example, Japanese Unexamined Patent Application Publication No. 2000-13106). In such second related arts, a coupled polarization line for directly coupling (hereinafter referred to as jump-coupling) the resonators, which are one or more stages away from each other, is provided in a casing covering the dielectric substrate or on the electrodes on the dielectric substrate to form attenuation peaks at both the high-frequency side and the low-frequency side of the passband.
In the above first related arts, for example, rectangular apertures are used to constitute the PDTL resonator in a dielectric resonator device. When the thickness, the permittivity, and the size of the cavity of the dielectric substrate are constant, the resonant frequency is determined by the length of the resonators. Since the length of the resonators is uniquely determined in accordance with the resonant frequency, an unloaded Q factor or spurious characteristics are determined only by the width of the resonators, thus decreasing the flexibility in design of the resonators.
In the above second related arts, the electrical length of the coupled polarization line for forming the attenuation peak is set to 180° or more. Accordingly, the spurious resonance of the coupled line for polarization can appear near the passband to deteriorate the attenuation characteristics.
In addition, since the level of the jump-coupling varies with variation in the distance between the coupled line for polarization and the resonators or in the electrical length of the coupled line for polarization, there is a problem in that the frequency of the attenuation peak is varied due to the positional shift or variation in size of the coupled line for polarization to destabilize the attenuation characteristics.
Furthermore, when the coupled line for polarization is formed on the same substrate as the resonators in order to lessen the influence of the positional shift or the like of the coupled line for polarization, it is necessary to sufficiently decrease the level of coupling between the coupled line for polarization and another resonator (for example, the second-stage resonator) although the coupled line for polarization is coupled to the resonators (for example, the first-stage and third-stage resonators) that are to be jump-coupled to each other. Hence, there is a problem in that the dielectric substrate tends to increase in size.